The invention relates to a method of making a shaped part having at least two regions with different microstructure and ductility from a semifinished part, in particular a blank of hardenable steel, by heating in a continuous furnace and then hardening, where the semifinished part is heated in the continuous furnace to austenitization temperature, then a first region of the semifinished part is cooled to a temperature at which the microstructure of the region is transformed into a ferritic-pearlitic microstructure while a second region of the semifinished part is maintained at austenitization temperature, and finally the semifinished part is deformed in a press hardener into the shaped part and tempered.
From EP 1 180 470 [U.S. Pat. No. 6,524,404], a B-pillar as body part for a motor vehicle is known that is designed in the form of a longitudinal profile having a first longitudinally extending region with a predominantly martensitic material microstructure and a strength above 1400 N/mm2 and a second longitudinally extending region of higher ductility with a predominantly ferritic-pearlitic material microstructure and a strength below 850 N/mm2.
This document describes a workpiece configured with properties desired in the automotive industry.
DE 19 743 802 [U.S. Pat. No. 5,972,134] describes a method of making a metallic shaped part for motor vehicle parts having regions with high ductility. In this method, a blank made of suitable steel is provided that, in certain regions intended in the finished part to have a higher strength than the rest of the part, are brought in a time of less than 30 seconds to a temperature between 600° C. and 900° C. Subsequently, the heat-treated blank is deformed in a press into a shaped part. The tempering is also carried out in the press.
In this document, another procedure is described, whereby a blank is first preshaped or finish-shaped by a molding process and then certain regions of the semifinished or shaped part are heat-treated in the above-described manner. The regions then have a significantly higher strength with respect to the rest of the part. The tempering can be carried out in the press with reduced or even without shaping. If necessary, only a repressing takes place. A further procedure is described in this document, whereby the whole blank is first homogeneously heated to a temperature between 900° C. and 950° C., deformed in a press into a shaped part and is subsequently tempered. After this, a specific partial increase of the ductility in desired regions of the shaped part is carried out by partial reheating.
Such a procedure is relatively complex.
DE 102 56 621 [U.S. Pat. Nos. 7,578,894 and 7,540,993] describes a further method of making such a shaped part. According to this document, the semifinished part to be heated, for example a blank or a preformed part, is conveyed during the transport through a continuous furnace that is separated in two different temperature zones so that in one region a relatively high temperature and in another region a relatively low temperature is provided to prepare the part for the subsequent shaping and hardening. Thereafter, this part is placed into a hot shaping and hardening press to generate the finished part with the appropriate microstructure.
In such a system it is difficult to form parts with first and second regions of different lengths because the separation of the temperature zones usually is done by providing in the continuous furnace a partition that can only be displaced with considerable effort to generate differently shaped parts.
DE 10 2006 017 317 [US 2007/0235113] describes a similar method. Here, a blank or a semifinished part is heated to austenitization temperature and subsequently placed in a shaping tool with a press. Shaping the semifinished and quenching the semifinished part is carried out by contact with the shaping tool. Here, regions of the semifinished part that have to transmit deep drawing forces during the shaping are cooled in a dosed manner after heating to a temperature above austenitization temperature and prior to contact of the appropriate regions with the shaping tool without reaching in the regions the cooling rate necessary for hardening. The semifinished part is subsequently deformed and hardened in the shaping tool.
This procedure is goal-oriented; however it is relatively complex and difficult to manage.
Finally, DE 10 2006 018 406 discloses a method of heating workpieces, in particular sheet metal parts provided for press hardening, where heat is supplied to the workpiece over a period of time to heat it to a predetermined temperature. To this end, heat is dissipated during the heating from a selected region of the workpiece so that the temperature reached during the heating period in the selected region stays below the predetermined temperature. This procedure too is complicated and energy-consuming because in some cases the energy applied has to be dissipated again right away.